In an optical disk apparatus, control must be performed so that a light beam which is used for recording or reproduction follows along a track which is provided on an optical disk. This control is called tracking control. It is also necessary to keep a constant convergence state of the light beam which is radiated onto the optical disk. This control is called focus control.
In recent years, optical disks of higher recording densities have been developed. As the recording density of an optical disk increases, the track pitch becomes narrower, and the pits and marks that are formed on a track become smaller in size. This makes it necessary to increase the accuracy of tracking control and focus control of the light beam, and various techniques therefor have been proposed.
A tracking error signal and a focus error signal are utilized for tracking control and focus control. A tracking error signal and a focus error signal are generated by, for example, radiating a light beam onto an optical disk, detecting the light reflected from the optical disk with a plurality of photodetectors, and ascertaining a difference between the outputs obtained from the plurality of photodetectors.
A tracking error signal and a focus error signal rely on the intensity of the light entering the photodetectors. Hence, when the reflectance of the optical disk varies, the sizes of amplitude of the tracking error signal and focus error signal will also vary. Therefore, the tracking control and focus control utilizing the tracking error signal and the focus error signal may possibly become unstable due to the changes in amplitude of these signals.
In order to solve this problem, Patent Document 1 discloses inputting a tracking error signal and a focus error signal to an automatic gain control circuit (AGC circuit), and performing control so that these signals will become constant in amplitude. For example, the AGC circuit divides a tracking error signal or focus error signal by a sum signal which is obtained by adding the signals obtained from the plurality of photodetectors. When the reflected light is weak, the amplitude of the sum signal indicating the intensity of the entire reflected light returning from the optical disk also becomes small. Therefore, with this method, the size of amplitude of the tracking error signal or focus error signal will be adjusted in accordance with the intensity of the reflected light, so that a tracking error signal and a focus error signal having substantially constant amplitude levels can be obtained.
Also studied are techniques which keep a constant amplitude of the tracking error signal or focus error signal by using constructions other than the AGC circuit. For example, Patent Document 2 proposes a method of using laser power to compensate for fluctuations in the size of amplitude. Patent Document 3 discloses a technique of rotating a wavelength plate inside an optical pickup to change the amount of birefringence, thus adjusting the amount of light received by the photodetectors.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 5-36143
[Patent Document 2] Japanese Laid-Open Patent Publication No. 5-101394
[Patent Document 3] Japanese Laid-Open Patent Publication No. 2001-351238